Field of the Invention
The present invention relates to a transmission, especially, a gear transmission.
Related Art
JP 2010-115969 A discloses a conventional four-wheel drive vehicle, e.g., a utility vehicle, including front and rear transaxles. The rear transaxle includes a transaxle casing incorporating a gear transmission that includes at least one gear train and a transmission output shaft driven by the gear train. A power take-off (hereinafter, PTO) unit includes a PTO input shaft, a PTO shaft, and a PTO gear train for transmitting power from the PTO input shaft to the PTO shaft. The PTO unit is attached to the rear transaxle so as to drivingly connect the PTO input shaft coaxially to the transmission output shaft. The PTO shaft is drivingly connected to the front transaxle, so that the rotary power of the transmission output shaft of the rear transaxle is transmitted to the front transaxle.
The vehicle includes a parking brake for braking axles drivingly connected to the gear train. The parking brake is disposed in a middle portion of the transaxle casing and on a transmission shaft of the transmission at the upstream of the transmission output shaft connected coaxially to the PTO input shaft. The parking brake at this position may be hidden by many surrounding parts. To access the parking brake, considerably many parts in the rear transaxle may have to be disassembled. Therefore, this position of the parking brake is inconvenient for accessing the parking brake for maintenance or another purpose.
JP S64-012956A may be considered as teaching an arrangement of a parking brake in a transmission casing of a four-wheel drive vehicle adjacent to a sidewall of the transmission casing. This position of the parking brake is convenient for accessing the parking brake. However, in the transmission casing, a transmission output shaft for driving rear axles is extended in the fore-and-aft direction of the vehicle, the parking brake is disposed at one of right and left sides of the transmission output shaft and includes a brake shaft extended parallel to the transmission output shaft, and a PTO shaft for driving front axles is disposed at the other of right and left sides of the transmission output shaft and is extended parallel to the transmission output shaft. A gear train is interposed between the transmission output shaft and the parking brake, and other speed-change gear trains are interposed between the transmission output shaft and the PTO shaft. Such many gear trains may reduce the brake force before it reaches the PTO shaft. If the vehicle is parked on a slope, the reduced braking force onto the PTO shaft and the front axles may cause the vehicle to unexpectedly descend the slope.
JP 2006-292144 A discloses another conventional transaxle incorporating a gear transmission. The gear transmission includes a transmission shaft on which at least one gear for selecting a driving speed or direction is provided rotatably relative to the transmission shaft. A gearshift clutch is provided on the transmission shaft. The gearshift clutch includes a clutch hub fixed on a transmission shaft, and a sleeve spline-fitted on the clutch hub. The clutch hub and the gear are formed on outer peripheral portions thereof with respective splines. The sleeve is formed on an inner peripheral portion thereof with splines. The sleeve is constantly spline-fitted to the clutch hub, and the sleeve is slidable in the axial direction of the clutch hub so as to be shiftable between a clutch-on position to engage its splines with the splines of the gear and a clutch-off positon to disengage its splines from the splines of the gear. The splines of the sleeve and the gear are formed with chamfers for smoothening meshing of the splines.
Such a gearshift clutch is advantageously compact in comparison with a synchromesh system. However, the gearshift clutch still has a problem of a slide lock of the sleeve such that the sleeve is hard to slide to mesh its splines with the splines of the gear because tips of the splines of the sleeve formed with the chamfers abut against those of tips of the splines of the gear formed with the chamfers. In this regard, the pitch of the splines of the clutch hub is the same as that of the splines of the gear, and the space between every adjoining splines of the clutch hub (and the gear) is substantially as wide in the peripheral direction of the transmission shaft as each spline of the sleeve, so that the sleeve cannot rotate relative to the clutch hub. If the difference in rotational position between the clutch hub and the gear causes that the splines of the clutch hub and the splines of the gear are arranged quite alternately in the peripheral direction of the transmission shaft, the spaces among the splines of the gears are hidden behind the splines of the clutch hub from the splines of the sleeve, and each of the splines of the gear blocks the spaces from each of the splines of the sleeve, thereby hindering the sleeve from further sliding toward the gear. This state is defined as the slide lock of the sleeve. The slide lock is kept before the difference in rotational position between the clutch hub and the gear is reduced to a level where the chamfers of the splines start to function properly.
The slide lock of the sleeve may often occur when a vehicle having been stationary with an engine stopped is started by starting the engine. An operator may have to repeat a shift operation of a manipulator for sliding the sleeve between the clutch-off position and the clutch-on position until the gear comes to an appropriate rotational position relative to the sleeve.
It is conceivable that the axial thrusting force for sliding the sleeve in the direction to the gear should be increased to force the slide of the sleeve against the gear. However, such an increased axial thrust force may shorten the life of component members of the gear clutch. Further, the increased force may increase a resistance of the gear when the meshing of the splines of the sleeve with the splines of the gear is completed as well as the resistance of the gear when the slide lock of the sleeve occurs, whereby the operator may feel the double resistances during the clutch-on operation of the sleeve to engage with the gear.
Moreover, JP 2004-082926A discloses another conventional four-wheel drive vehicle, e.g., a utility vehicle, provided with a parking brake. This parking brake is disposed in a rear transaxle drivingly connected to a power unit including an engine and a transmission. This parking brake is a multi-friction disc type brake, which needs a large space for sufficiently lubricating the friction discs. A pawl engaging type brake may be considered as an alternative parking brake adaptable to the vehicle because it is needs no lubrication fluid supply and is compact, however, such a brake having pawls needs a cam which may have to be disposed stably in the vehicle so as to complicate design of the vehicle.